The essential function of mitosis is the delivery of a complete set of chromosomes to each daughter cell. This requires attachment of spindle microtubules to kinetochores, specialized structures formed at centromeres. Kinetochores are more than just structural elements that mediate faithful chromosome inheritance. The unattached kinetochore is also the signal generator for the mitotic checkpoint (also known as the spindle assembly checkpoint), the major cell cycle control mechanism in mitosis. Each unattached kinetochore generates a wait anaphase inhibitor that blocks destruction of cyclin B and securin, thereby blocking disjoining of duplicated sister chromosomes and advance into anaphase. We have previously identified a kinetochore-bound microtubule-dependent motor, CENP-E; to be the target of an Aurora kinase- protein phosphatase I (PP1) switches that regulates congression of initially misoriented chromosomes. We will now determine how this switch modulates CENP-E motor activity and the role of PP1-bound CENP-E in reactivating capture of spindle microtubules by kinetochores of initially misaligned, polar chromosomes. A major effort will use all purified components to reconstruct mitotic checkpoint signaling, and with this we will determine the mechanism for its selective inhibition of ubiquitination of cyclin B and securin, as well as the roles played by essential checkpoint kinases. Centrosomes are the major microtubule organizing centers of animal cells and play a particularly important role during mitosis, where they form the poles of the bipolar microtubule spindle upon which chromosomes are segregated. The duplication of the centrosome is tightly controlled, and extra centrosomes can cause errors in spindle formation that lead to subsequent chromosome missegregation. Supernumerary centrosomes are present in many types of cancers and can be found in early premalignant lesions. More than 100 years ago the great cytologist Boveri proposed a link between tumorigenesis and aneuploidy, including the still untested contribution of centrosome amplification. Using a conditional mouse model(s) in which extra centrosomes can be induced, we will determine whether centrosome amplification 1) promotes cellular transformation, 2) the formation of spontaneous tumors, 3) is capable of facilitating the development of carcinogen-induced tumors, and 4) is able to accelerate the development (or increase the aggressiveness or metastatic potential) of tumors driven by the loss of a tumor suppressor gene. Finally, we will utilize gene-targeting and replacement in cultured human cells to determine how a key upstream regulator of centrosome duplication, Polo-like kinase 4, acts to limit centrosome duplication to once per cell cycle.